foreach ($themes as $theme) {
        if ($theme->getCode() == $themeName) {
            $this->config->assignToStore(
                $theme,
                [Store::DEFAULT_STORE_ID],
                ScopeConfigInterface::SCOPE_TYPE_DEFAULT
            );
        }
    }

$design = $this->objectManager->create('MagentoThemeModelViewDesign');
$design->setDesignTheme($themeName, 'adminhtml');